Deadly clashes broke out at the Ain el-Hilweh camp last month after Islamist gunmen tried to assassinate Mahmoud Khalil, a leader of Palestinian political faction Fatah, forcing hundreds to flee.
"The Agency does not tolerate actions that breach the inviolability and neutrality of its installations," United Nations Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) said in a statement.
It said that schools in the camp were unlikely to be available for 3,200 children at the start of the new school year.
"UNRWA reiterates its call on armed actors to immediately vacate its facilities, to ensure unimpeded delivery of much needed assistance to Palestine Refugees," the agency added.
Some 400,000 refugees live in Lebanon's 12 Palestinian camps, which date back to the 1948 war between Israel and its Arab neighbors.
Established in 1949 following the first Arab-Israeli war, UNRWA provides public services including schools, primary healthcare and humanitarian aid in Gaza, the West Bank, Jordan, Syria and Lebanon.
(Reporting by Gabrielle Tétrault-Farber; Editing by Devika Syamnath)

A general view of Ain al-Hilweh Palestinian refugee camp during clashes between supporters of the Fatah movement and rival groups, in Sidon, Lebanon, 01 August 2023. The death toll has risen to at least 11, Dorothee Klaus, Director of UNRWA Affairs in Lebanon said. The clashes began on 29 July at Ain al-Hilweh camp forcing thousands of Palestinians to flee. EPA-EFE/WAEL HAMZEH